Alloy pipes and methods of making same

Abstract

A profiled tube ( 50 b ) comprising a high carbon content alloy (greater than 0.3%), superalloy or other high creep strength material, has a fluted bore with internal fins ( 52 b ) and intervening valleys or troughs ( 56 b ). Such a tube is made by centrifugal casting and subsequent electrochemical machining by drawing of an electrode ( 104 ) along the tube while at the same time passing electrolyte along the tube and around the electrode and also applying an electrical current between the tube and electrode, so that material of the tube passes into solution in the electrolyte. The electrode ( 104 ) has the same profile as the end-profile required in the tube ( 80 ) but has an inclined working face ( 124 ). An electrode rod ( 92 ) drives the electrode from one end mounting means ( 82 ) to another ( 84 ), each of which support the tube ( 80 ) and supply/exhaust electrolytic solution. The electrolyte is preferably sodium nitrate, although an acid based system is feasible.

1. A method of forming an end-profile in the bore of a tube of creep resistant alloy, the method comprising:
 drawing, from a first end of the tube to a second end of the tube, an electrode having an external profile corresponding to a desired end-profile of the tube;  
 applying, across a space between a target area of the tube and the electrode, a potential difference for driving an electric current between the tube and the electrode;  
 passing an electrolyte along the tube to enable the electric current to flow and to cause metal from the surface of the target area of the tube to enter solution in the electrolyte;  
 maintaining the rate of draw of the electrode along the tube so that the space between the target area of the tube and the electrode remains substantially constant; and  
 selecting the creep resistant alloy to include the following constituents in percent by weight as indicated: carbon, 0.1-0.5; chromium, 20-35; nickel, 20-45; niobium, 0-2; tungsten, 0-5; additions, 0-1; and the balance containing iron.  
 
     
     
       2. The method of  claim 1 , wherein applying a potential difference comprises selecting a potential difference such that, over the target area, the electric current has a current density between 20 and 80 amps cm-2. 
     
     
       3. The method of  claim 1 , wherein applying a potential difference comprises selecting a potential difference such that, over the target area, the electric current has a current density between 30 and 70 amps cm-2. 
     
     
       4. The method of  claim 1 , wherein maintaining the rate of draw comprises selecting a rate of draw between 2 and 20 millimeters per minute. 
     
     
       5. The method of  claim 1 , wherein maintaining the rate of draw comprises maintaining the rate of draw such that the space is between 0.2 and 0.7 millimeters. 
     
     
       6. The method of  claim 1 , wherein passing an electrolyte comprises selecting the electrolyte to be an aqueous solution of an inorganic salt. 
     
     
       7. The method of  claim 6 , further comprising selecting the salt from the group consisting of a nitrate, a chloride of sodium, a bromide of sodium, a chloride of potassium, a bromide of potassium. 
     
     
       8. The method of  claim 7 , further comprising maintaining the salt at a temperature of between 35 and 45 C, and/or at a specific gravity between 1.1 and 1.25, and/or at a pH of between 8 and 10. 
     
     
       9. The method of  claim 8 , further comprising maintaining the salt at a temperature of between 38 and 42 C, and/or at a specific gravity of about 1.18, and/or at a pH between 8.5 and 9.5. 
     
     
       10. The method of  claim 8 , wherein passing an electrolyte comprises selecting the electrolyte to include a mineral acid. 
     
     
       11. The method of  claim 10 , further comprising selecting the mineral acid from the group consisting of sulphuric, nitric and hydrochloric acids. 
     
     
       12. The method of  claim 6 , further comprising selecting the salt to be sodium nitrate. 
     
     
       13. The method of  claim 1 , further comprising providing a tube having a start-profile having a maximum diameter less than the diameter of a largest circle capable of fitting in the end-profile. 
     
     
       14. The method of  claim 1 , further comprising inclining the electrode with respect to the direction of draw, so that the target area of the tube is increased. 
     
     
       15. An electrochemical machining apparatus for machining an end-profile on the inside of a tube having a start-profile, the apparatus comprising:
 a starting mount and an ending mount for mounting the tube, each being adapted to seal against one end of the tube and to supply one end of the tube with electrolyte;  
 an insulated conductive rod extending through an aperture in one of the starting and ending mounts;  
 an electrode having a profile corresponding to the end-profile and mounted on a first end of the insulated conductive rod;  
 a power source providing an electrical voltage between the tube and electrode;  
 a draw operatively connected to a second end of the rod to drive the electrode into the bore of the tube;  
 the rod being long enough that the electrode can be within the confines of either mount without the draw contacting the other of the mounts  
 wherein the end-profile includes a plurality of peaks, a plurality of troughs adjacent to each of the peaks, and a plurality of undulating sections connecting each of the peaks with an adjacent trough; and  
 the electrode further includes, at a front end thereof, a plurality of insulated sections, the insulated sections being disposed to suppress, during machining of the tube, removal of material from the peaks of the end profile.  
 
     
     
       16. The apparatus of  claim 15 , wherein the end-profile comprises a non-circular end profile having, in cross-section, a length that is at least 10% longer than the circumference of the smallest circle encompassing the entire end-profile. 
     
     
       17. The apparatus of  claim 16 , wherein the electrode comprises a front end and a rear end, the rear end having a final section of constant cross-section. 
     
     
       18. The apparatus of  claim 17 , wherein the final section comprises a section less than 2 millimeters in length. 
     
     
       19. The apparatus of  claim 18 , wherein the cross-section of the front end has an overall diameter less than or equal to a minimum diameter of the start-profile of the tube and the electrode tapers from the rear end to the front end. 
     
     
       20. The apparatus of  claim 15 , wherein the electrode further comprises, at a rear end thereof, an uninsulated section for removal of material from the complete circumference of the tube bore. 
     
     
       21. The apparatus of  claim 15 , wherein the electrode comprises:
 a front guide having a cross-section corresponding to the start-profile of the tube;  
 a rear guide having a profile corresponding to the end-profile of the tube; and  
 a passage for enabling flow of electrolyte along the tube past the electrode.  
 
     
     
       22. The apparatus of  claim 21 , wherein the end-profile includes peaks and troughs, and wherein the rear guide is shaped to provide a close sliding fit to the peaks of the end-profile and to form the passage between spaces bounded by troughs of the end-profile. 
     
     
       23. The apparatus of  claim 21 , wherein the front guide includes slots in its surface, the slots defining the passage. 
     
     
       24. The apparatus of  claim 15 , wherein the rod extends through the starting mount. 
     
     
       25. The apparatus of  claim 24 , further comprising an insulated rod extension connected to the power source and to the electrode and passing through an aperture in the end mount. 
     
     
       26. The apparatus of  claim 15 , wherein the tube comprises creep resistant alloy. 
     
     
       27. The apparatus of  claim 26 , wherein the creep resistant alloy comprises the following constituents in percent by weight as indicated: carbon, 0.1-0.5; chromium, 20-35; nickel, 20-45; niobium, 0-2; tungsten, 0-5; additions, 0-1; and the balance containing iron. 
     
     
       28. The apparatus of  claim 26 , wherein the creep resistant alloy has a mean 100,000 hour rupture value of more than 6 MPa at 1000 C. 
     
     
       29. A method of forming a furnace tube, the method comprising:
 providing a molten creep resistant alloy;  
 casting the alloy in a rotating tubular mold to form a tubular blank having a central bore; and  
 electrochemically machining a non-circular profile inside the bore.  
 
     
     
       30. The method of  claim 29 , wherein electrochemically machining comprises drawing a correspondingly shaped electrode along the tube while passing electrolyte along the tube. 
     
     
       31. The method of  claim 29 , further comprising boring the blank to form a circular bore of predetermined radius. 
     
     
       32. The method of  claim 29 , wherein electrochemically machining comprises:
 providing an electrode having an external profile corresponding to a desired end-profile of the tube;  
 drawing the electrode from a first end of the tube to a second end of the tube;  
 applying a potential difference across a space between the tube and the electrode;  
 passing electrolyte between the tube and the electrode; and  
 maintaining the rate of draw of the electrode so as to maintain an extent of the space.  
 
     
     
       33. The method of  claim 29 , wherein electrochemically machining is carried out on electrochemical machining apparatus comprising:
 a starting mount and an ending mount for mounting the tube, each being adapted to seal against one end of the tube and to supply one end of the tube with electrolyte;  
 an insulated conductive rod extending through an aperture in one of the starting and ending mounts;  
 an electrode having a profile corresponding to the end-profile and mounted on a first end of the insulated conductive rod;  
 a power source providing an electrical voltage between the tube and electrode;  
 a draw operatively connected to a second end of the rod to drive the electrode into the bore of the tube;  
 the rod being long enough that the electrode can be within the confines of either mount without the draw contacting the other of the mounts.  
 
     
     
       34. The method of  claim 29 , wherein providing the molten creep resistant alloy comprises providing an allow having the following constituents in percent by weight as indicated: carbon, 0.1-0.5; chromium, 20-35; nickel, 20-45; niobium, 0-2; tungsten, 0-5; additions, 0-1; and the balance containing iron. 
     
     
       35. The method of  claim 29 , wherein providing a molten creep resistant alloy comprises providing an alloy that, when solidified, has a mean 100,000 hour rupture value of more than 6 MPa at 1000 C. 
     
     
       36. A method of forming an end-profile in the bore of a tube of creep resistant alloy, the method comprising:
 drawing, from a first end of the tube to a second end of the tube, an electrode having an external profile corresponding to a desired end-profile of the tube;  
 applying, across a space between a target area of the tube and the electrode, a potential difference for driving an electric current between the tube and the electrode;  
 passing an electrolyte along the tube to enable the electric current to flow and to cause metal from the surface of the target area of the tube to enter solution in the electrolyte;  
 maintaining the rate of draw of the electrode along the tube so that the space between the target area of the tube and the electrode remains substantially constant; and  
 selecting the creep resistant alloy to have a mean 100,000 hour rupture value of more than 6 MPa at 1000 C.  
 
     
     
       37. The method of  claim 36 , wherein applying a potential difference comprises selecting a potential difference such that, over the target area, the electric current has a current density between 20 and 80 amps cm-2. 
     
     
       38. The method of  claim 36 , wherein applying a potential difference comprises selecting a potential difference such that, over the target area, the electric current has a current density between 30 and 70 amps cm-2. 
     
     
       39. The method of  claim 36 , wherein maintaining the rate of draw comprises selecting a rate of draw between 2 and 20 millimeters per minute. 
     
     
       40. The method of  claim 36 , wherein maintaining the rate of draw comprises maintaining the rate of draw such that the space is between 0.2 and 0.7 millimeters. 
     
     
       41. The method of  claim 36 , wherein passing an electrolyte comprises selecting the electrolyte to be an aqueous solution of an inorganic salt. 
     
     
       42. The method of  claim 41 , further comprising selecting the salt from the group consisting of a nitrate, a chloride of sodium, a bromide of sodium, a chloride of potassium, a bromide of potassium. 
     
     
       43. The method of  claim 42 , further comprising maintaining the salt at a temperature of between 35 and 45 C, and/or at a specific gravity between 1.1 and 1.25, and/or at a pH of between 8 and 10. 
     
     
       44. The method of  claim 43 , further comprising maintaining the salt at a temperature of between 38 and 42 C, and/or at a specific gravity of about 1.18, and/or at a pH between 8.5 and 9.5. 
     
     
       45. The method of  claim 43 , wherein passing an electrolyte comprises selecting the electrolyte to include a mineral acid. 
     
     
       46. The method of  claim 45 , further comprising selecting the mineral acid from the group consisting of sulphuric, nitric and hydrochloric acids. 
     
     
       47. The method of  claim 41 , further comprising selecting the salt to be sodium nitrate. 
     
     
       48. The method of  claim 36 , further comprising providing a tube having a start-profile having a maximum diameter less than the diameter of a largest circle capable of fitting in the end-profile. 
     
     
       49. The method of  claim 36 , further comprising including the electrode with respect to the direction of draw, so that the target area of the tube is increased.